Existing technologies for indirect calorimetry and resting energy expenditure monitoring are typically based on electron paramagnetic resonance, electrochemical and infrared detection for detection of oxygen consumption rate and carbon dioxide production rate. The electron paramagnetic resonance method is humidity dependent, the electrochemical detection face challenging lifetime issues, and the infrared detection are prone to interference and fall short in selectivity and specificity. In addition, the cost of the analyzer combining these different detection principles in a single integrated device is expensive.1 Even though a CPT insurance code has been established for using these technologies, the high cost inherent to these technologies prohibits them from reaching a larger consumer market.
Recently, other respiratory analyzers have been developed. One is for exercise use, which includes a single oxygen sensor and a wind guard.2 Although this analyzer allows for assessment of oxygen consumption rate for sport activities, it does not detect carbon dioxide production rate, which is necessary for accurate detection of energy expenditure. It has been established that accurate assessment of energy expenditure and respiratory quotient requires one to detect both oxygen consumption and carbon dioxide production rates.1 A wireless wearable mask including both oxygen and carbon dioxide sensors has been disclosed.3 The sensors are two separated pieces, one detects oxygen, and the other one detects carbon dioxide, and the two pieces are based on different sensing principles. While the oxygen sensor is based on galvanic fuel cell detection, the carbon dioxide sensor is based on infrared detection using a concave-wall and reflective-surface. Although the analyzer can detect both oxygen and carbon dioxide, the use of different detection principles and separated pieces adds complexity to the system, making it expensive and bulky. Furthermore, the galvanic fuel cell for oxygen detection faces the limitation of electrochemical techniques mentioned above.
One publication describes simultaneous detection of oxygen and carbon dioxide using a single detection principle.4 The system is based on the detection of fluorescence light emitted from fluorophore molecules upon excitation, typically UV or high-energy light. It has been applied to monitor carbon dioxide and oxygen for micro-organism cultures, but not for analysis of metabolites in breath. Fluorophores are prone to humidity and temperature changes, so the approach may not be suitable for detection of oxygen and carbon dioxide in breath. In addition, the fluorescence detection faces photo-bleaching issue, requires low noise and sensitive photodetector, and UV light source, which make it undesirable for a low cost and miniaturized device.
Acetone is another metabolite that is indicative of fat burning. Several devices have been disclosed related to measuring acetone. Some of them are based on electrochemical5-7 and electrical8-10 measurements.
One example of electrochemical detection uses enzymes.5-7 Such devices face stability challenges and require controlled humidity conditions.11 Examples of existing electrical sensors are based on metal-oxide devices,8-9 or nanoparticle devices.10 Unfortunately the metal-oxide devices require high temperatures during operation, leading to high power consumption. Similar difficulties are presented by the nanoparticle devices in that they require pattern recognition algorithms which are difficult to implement in complex changing sample matrixes such as when monitoring breathing.
Another acetone apparatus has been disclosed for metabolic fitness training.12 The device provides only a qualitative measure of acetone levels of maximum fat burn rate. An additional limitation of the device is that it does not detect oxygen and carbon dioxide, which are needed for energy expenditure and respiratory quotient assessment. Yet another acetone apparatus for diabetic diagnosis has been disclosed.13 The apparatus employs a microplasma source in combination with a spectrometer. The microplasma approach requires bulky instrumentation, high power to produce excited acetone fragments from the breath gas, and it is difficult to miniaturize.
The metabolic analyzer disclosed here for the first time overcomes sensitivity, selectivity, stability, cost and power consumption problems found in known devices and systems. In contrast to known devices and systems, the instant disclosure describes a new and novel metabolic analyzer based on the detection of several metabolic signatures via distinct color changes of sensing materials coated onto a solid support. Each sensing material is designed such that they interact and react specifically with each metabolic analytes, including oxygen, carbon dioxide, acetone and other metabolites. These sensing materials can be deposited on the same support to create an array such that each sensor in the array detects specifically one metabolite. In comparison to fluorescence detection schemes that measure weak emission of light,4 the color detection apparatus in the present disclosure measures absorption of light, which requires neither low noise and sensitive photodetectors, nor UV light sources.
A basic configuration of the metabolic analyzer detects at least both oxygen and carbon dioxide, which allows for indirect calorimetry that evaluates a person's energy expenditures (kcal/day) from the rates of consumed oxygen and produced carbon dioxide in breath. The analyzer also provides respiratory quotient (RQ) from the ratio of oxygen to carbon dioxide, which indicates the type of food substrate metabolized, and or the breathing status under an aerobic or anaerobic exercise condition. Such a capability will benefit the large and growing obese and overweight population, and also provide more effective training of athletes and armed forces. Unlike physical activity monitoring devices, such as accelerometers, which cannot monitor resting energy expenditures, the instant invention's indirect calorimeter specifically targets resting energy expenditure. This is important because over 75% of a person's energy expenditure is resting energy.14 
In another advance over existing techniques and devices also disclosed here for the first time is a ketone (for example, acetone) detection capability built into the metabolic analyzer. Acetone level measurements provide extra information about metabolism and can discriminate fat vs. carbohydrates burning. The energy expenditure, together with acetone detection capability, provides additional values for more effective weight loss and control, and physical training programs.
In brief, the novel metabolic analyzer disclosed hereinbelow can measure Energy Expenditure (EE) and Respiratory Quotient (RQ). The EE quantifies the amount of calories consumed by the body either at resting state (Resting Energy Expenditure, REE), or during an activity (office work, work bench, computer work, etc). The RQ determines the type of dominant food substrate metabolized by the body. Both parameters are calculated from the measurement of consumed oxygen rate and produced carbon dioxide rate. The novel metabolic analyzer disclosed here for the first time enables more effective weight management and fitness applications as described below.
Various methods for weight and fitness management have been developed and practiced. Some methods include use physical sensors, such as accelerometers, to evaluate the energy expenditure of a person during exercise. However, exercise activities represent only a small percentage (<15%) of the person's energy expenditure averaged in a day15 Other approaches consider more accurate strategies, including actual exercise, and calories intake to forecast weight changes.16,17 Although these approaches are more accurate, they still lack of the determination of the major component of energy expenditure on a day, the resting energy expenditure, which not only enables more accurate determination of total energy expenditure (TEE), but also more importantly an indication of the metabolic stage of the person's body during a weight loss or fitness plan.
In order to overcome the problem, methods including metabolic rate measurement (e.g. resting energy expenditure) have been proposed. One method involves measuring metabolic rate and body composition using a plethysmographic air chamber, magnetic resonance imaging or computed tomography.18 The method also includes consultation of a nutritional counselor. The counselor evaluates the metabolic rate and body composition, provides advice for weight management and determines the weight goal accomplishments. In addition, the method includes a massage therapy reward if the person reaches the goal. Although the method is complete, and accurate, the use of bulky instrumentation for assessment of body composition precludes the implementation of the method at the person's home, office or ordinary living physical places.
In a separate approach, the use of a portable indirect calorimeter is proposed to obtain resting metabolic rate, and data of food intake and activities are used as a method of health management plan.19 Although the method includes free-living conditions measures, the use of data of food intake makes the method cumbersome. It is has been well established that it is difficult keep accurate tracking of food intake, and these methods lead to problems of under-reporting.
More recent publications have recognized the problem of food intake data, and proposed alternative approaches. One such approach is a method comprising energy expenditure, and modeling.20 The model is used to predict a weight value at a predetermined period of time. The prediction can be made based on energy expenditure and an initial weight measure. Although the method is accurate, it focuses on prediction of weight at a fixed period of time, and precludes any additional outcome or recommendation at non-fixed time periods, or new weight values (which could include target weights). Another system proposes the assessment of unambiguous food energy intake via the assessment of metabolic rate and body composition change.21 Although the system may be accurate, it requires measuring at least two parameters, body composition, and energy expenditure, each of which currently requires dedicated devices. On the other hand the assessment of body composition is complex, and still needs development of more accurate portable measurement devices.
Instead of focusing on measuring parameters that are either inaccurate or difficult to track (like food intake) or misrepresentative (like physical activity), the present invention focuses on key parameters that are meaningful yet easily and accurately measurable. These parameters include weight, REE and RQ of a person. Weight can be readily measured with various commercial devices, and REE and RQ can be measured with the metabolic analyzer disclosed in our prior application. Recommendations on diet and physical exercises are made based on the values and changes of the weight, REE and RQ, and the person's weight and fitness goal. The method may also include sensors that track physical activity-energy expenditure to provide total energy expenditure information, and imaging or video processing of the person's progress of a weight and fitness program.